New Mombasa was a futuristic metropolis in the East African Protectorate on Earth. It was quite advanced, and featured a mammoth space elevator in the center of the city, which was a landmark for miles around. It bordered the smaller city of Old Mombasa, which is where the Halo 2 level Outskirts took place.
New Mombasa is featured in Halo 2 as the only place the Covenant invasion force lands. The level Outskirts deals with Master Chief's attempt to fight his way through to New Mombasa, where the Covenant force is located. After arriving, the Master Chief finds a Covenant Scarab walker terrorizing the city, wrecking any attempt at Marine resistance. The Master Chief and a company of Marines followed this behemoth through the city, eventually boarding it and taking it down near the ocean. At this point the Chief boards a Pelican transport back to the In Amber Clad. The Prophet of Regret's carrier, its ground forces overrun, opens a slipspace rift which it exits through followed by the In Amber Clad. The resulting shock wave from the slipspace rift destroys most of the city and brings down the mammoth space elevator- pieces of it can be seen all across the savanna in the Halo 3 level "Tsavo Highway".
In Halo 3, the Covenant have returned to Earth, and have begun excavating the area where New Mombasa used to be. A giant crater is being excavated by Covenant Carriers, and the Prophet of Truth's Dreadnought sits in the center of a massive Forerunner artifact. This artifact was buried under the site of New Mombasa 100,000 years ago by the Foreunners. It is a trans-dimensional portal to the Ark, which the Forerunner know as The Librarian, used to transport Earth's native species to the Ark before the Flood forced the Forerunner to activate the Halo installations. Just as the UNSC forces launch a strike against the Covenant forces in the area, the portal opens and Truth's ships head through to the Ark. Nothing remains of New Mombasa except the portal, which is now closed, following the events of Halo 3.
Halo 3: ODST
Halo 3: ODST, the upcoming new campaign, is at least initially set in New Mombasa directly after the escaping carrier wreaks havoc on the city. The game's lead protagonist, a UNSC Orbital Drop Shock Trooper named " The Rookie," will have to make his way through the Covenant-occupied city, meet up with his separated allies, and escape.
New Mombasa is based upon the large Kenyan city of Mombasa, on the east side of the country. Mombasa is renowned for being the leading Kenyan tourist spot, while maintaing a healthy shipping industry that gains a large chunk of the country's yearly GDP.